Internal combustion engines operate according to a combustion cycle having four strokes of operation: intake, compression, combustion, and exhaust. In association with the intake stroke, an intake valve is timed to open and close, in synchronization with a piston. For example, during the intake stroke, the piston is configured to move from top dead center to bottom dead center within the cylinder. Furthermore, during the intake stroke, the intake valve opens and closes according to the movement of the piston to allow the air/fuel mixture from the intake passage to be drawn through the intake port and into the combustion chamber.
If the internal combustion engine is operating at low speeds, the intake valve would ideally be opened when the piston is at top dead center and closed when the piston is at the bottom dead center. However, for the internal combustion engine to operate at high speeds, the valve timing must be changed to allow a maximum amount of air/fuel mixture from the intake passage to enter the combustion chamber. To that end, the intake valve must be opened before the piston reaches top dead center and closed after the piston reaches bottom dead center. At high speeds, the earlier opening and later closing of the intake valve insures that a maximum amount of air/fuel mixture enters the combustion chamber.
To illustrate, when the intake valve is opened before the piston reaches top dead center, the intake valve and the exhaust valve are simultaneously in the open position. At high speeds, such “overlap,” created after the intake valve is initially opened and before the exhaust valve is finally closed, allows the exhaust gases exiting via the exhaust valve to act as a “siphon,” and draw the air/fuel mixture into the combustion chamber from the intake passage. Most importantly, the overlap also allows the air/fuel mixture remaining in the intake passage to accelerate up to speed before the piston begins the intake stroke.
In practice, if the internal combustion engine is operating at high speeds, and the intake valve is instead timed to open when the piston reaches top dead center, then a significantly smaller amount of fuel/air mixture could be drawn into the combustion chamber. For example, regardless of the position of the piston, the intake passage is charged with a static amount of air/fuel mixture before the intake valve is opened. However, when the internal combustion engine is operating at high speeds, the air/fuel mixture has a limited time to enter the combustion chamber during the intake stroke. Therefore, the air/fuel mixture must have sufficient speed to pass through the intake port and enter the combustion chamber before the piston begins the intake stroke. The earlier opening of the intake valve allows the air/fuel mixture to accelerate up to speed, thereby allowing an increased amount of air/fuel mixture to enter the combustion chamber when the internal combustion engine is operating at high speeds.
Furthermore, when the internal combustion engine is operating at high speeds, the later closing of the intake valve also allows an increased amount of air/fuel mixture to enter the combustion chamber. As discussed hereinabove, during the intake stroke, the air/fuel mixture is drawn through the intake port and into the combustion chamber from the intake passage. Furthermore, when the internal combustion engine is operating at high speeds, the speed of the air/fuel mixture entering the combustion chamber generates a significant amount of momentum. However, if the intake valve were instead timed to close when the piston reaches bottom dead center, the momentum of the air/fuel mixture remaining in the intake passage would be sacrificed. In fact, an appreciable amount of the air/fuel mixture would be prevented from entering the combustion chamber. Therefore, even though the piston is beginning the compression stroke, the later closing of the intake valve allows the above-described momentum to “carry” the air/fuel mixture remaining in the intake passage into the combustion chamber. Consequently, when the internal combustion engine is operating at high speeds, the earlier opening and later closing of the intake valve allows a maximum amount of air/fuel mixture to enter the combustion chamber.
However, at low speeds, the earlier opening and later closing of the intake valve are undesirable. For example, as discussed hereinabove, when the intake valve is opened before the piston reaches top dead center, the intake valve and the exhaust valve are both in the open position. But, when the internal combustion engine is operating at low speeds, the siphoning-effect during such overlap is relatively weak, and the air/fuel mixture is not drawn into the combustion chamber by the exiting exhaust gases. In fact, because the air/fuel mixture is not drawn into the combustion chamber before the piston reaches top dead center, the exhaust gases will instead be forced through the intake port and into the intake passage. These exhaust gases dilute the air/fuel mixture in the intake passage, and therefore, effectively decrease the amount of air/fuel mixture drawn into the combustion chamber.
In addition, when operating at low speeds, the later closing of the intake valve is also undesirable. As discussed hereinabove, when the internal combustion engine is operating at high speeds, the later closing of the intake valve allows momentum to carry the air/fuel mixture remaining in the intake passage through the intake port and into the combustion chamber. However, at low speeds, the air/fuel mixture does not have enough speed to generate the momentum necessary to carry the air/fuel mixture remaining in the intake passage into the combustion chamber. In fact, at low speeds, when the piston reaches position bottom dead center, the movement of the air/fuel mixture through the intake port effectively stops. Because the intake valve remains open when the piston is beginning the compression stroke, the piston forces an appreciable amount of air/fuel mixture in the combustion chamber back through the intake port, and therefore, effectively decreases the amount of air/fuel mixture drawn into and trapped in the combustion chamber.
Therefore, at low speeds, the earlier opening and later closing of the intake valve decreases amount of air/fuel mixture drawn into the combustion chamber. The net effect of this decrease is a reduction of the compression ratio of the internal combustion engine, and a decrease in the of amount torque of the internal combustion engine can produce at low speeds.
The amount of power produced by the internal combustion engine is determined by multiplying the speed of the internal combustion engine (measured in revolutions per minute, or R.P.M.) by the amount of torque the internal combustion engine can produce at that speed. Therefore, when compared to the amount of power produced by an internal combustion engine operating at low speeds where the intake valve opens at top dead center and closes at bottom dead center, the speed of the above-described internal combustion engine must be increased to produce the same amount of power. An increase in speed also increases of the amount of air/fuel mixture, and, therefore increases the fuel that must be consumed.
Because the timing of the piston and the intake valve is synchronized according to the combustion cycle, but not to the speed of the internal combustion engine, to accommodate variable speeds of internal combustion engines, compromises are necessarily made in the timing. For example, to accommodate high speeds, the intake valve can be timed to open significantly before the piston reaches top dead center and close significantly after the piston reaches bottom dead center. Furthermore, to accommodate low speeds, the cam shaft can be configured to provide for the rapid opening and closing of the intake valve in an attempt to minimize the above-discussed undesirable effects of the earlier opening and later closing of the intake valve. However, the rapid opening and closing of the intake valve increases the stress on the operation of the intake valve operating components, and thereby decreases the reliability of the internal combustion engine.
As a result, there is a need for an internal combustion engine where the above-discussed undesirable effects of the earlier opening and the later closing of the intake valve can be eliminated at low speeds without the need for the rapid opening and closing of the intake valve, and the associated increase of stress on the intake valve operating components.